1. Field of the Invention
The present invention is in the field of heart valve implantation. More particularly, the present invention is directed to a novel external support or frame designed to be affixed on the exterior of a natural sigmoid aortic or pulmonary valve root or a manufactured valved conduit made of biological material. The primary function of the external support frame is to maintain the appropriate geometry of the sigmoid valve complex which includes leaflets and sinuses of Valsalva, particularly at the time of surgical implantation of the complex. The present invention is also directed to the design and process of manufacture of the valved conduit made of biological membranes, in combination with the external support frame.
2. Brief Description of the Prior Art
The outflow tract of the right ventricle of the human heart comprises a muscular part situated in the outlet of the right ventricle, the pulmonary sigmoid valve and the beginning or origin of the pulmonary artery. This tract is often abnormal because of a congenital heart defect, or in some patients with prior cardiac surgery the tract is absent because it has been used in surgery to transfer the pulmonary valve of the patient into the aortic position as an aortic replacement. The just noted surgical operation, also called "switch" or the "Ross Procedure", is becoming increasingly popular since it results in a living valve in the aortic position as is described for example by Ross D. in Eur. J. Cardiothorac Surg 1992;6:113. The end result of this operation is that the patient, like the patient having a congenitally underdeveloped outflow tract, suffers from a lack of continuity between the right ventricle and the pulmonary artery. Furthermore, some complex congenital heart malformations are also presently treated by surgically by-passing the hypoplastic right ventricle with a conduit.
In order to bridge the gap between the right ventricle and the pulmonary artery in the above-summarized cases, several solutions have been suggested and used. These include the use of pulmonary or aortic homografts obtained from a cadaver (Hawkins J. A., J. Thoraic Cardiovasc Surg. 1992;104:910), heterografts obtained from animals (Edwards W. D., Arch. Pathol. Lab. Med. 1983; 107:400), patches of different materials with or without a single cusp, and cylindrical tubes made of different materials including bovine pericardium (Urrea Ms. Texas Heart J. 1993; 20:271).
All of the foregoing prior art solutions are laden with problems or serious risks either immediately after their implantation or thereafter. Kinking and partial obstruction of the implanted conduit, after the chest is closed, and lack of pulmonary valve competence (which always occurs with patches) can complicate the early post-operative period. Long term problems of durability are encountered often. These include calcification of the homografts and bioprosthesis, particularly in children (see Kirklin J. W. Ann. Thorac. Surg. 1987; 44:598), and progressive stenosis due to peel formation in the Dacron conduits (see Jonas R. A. Circulation 1985;72 (Suppl 2):77).
In view of the foregoing, there is a need to develop a new type of valved conduit to be used in the right side of the heart. The present invention represents a significant advance in this direction.
Moreover, human (homologous) and animal (heterologous) aortic and pulmonary valves are still used in the state-of-the-art for transplantation into patients suffering from aortic or pulmonary disease. Initially in the prior art these valves were dissected so that only the three leaflets were left with a small rim of arterial wall and myocardium used for suturing the replacement valve to the patient's own tissue. More recently, however there has developed an awareness in the art that the normal functioning of the sigmoid valve requires not only the three leaflets but also the three sinuses of Valsalva. In fact, it is now understood that the physiological sigmoid valve is a complex which includes the ventricular outflow tract, valve annulus, leaflets and sinuses of Valsalva with their distal limit of the supraaortic ridge or sino-tubular junction. Because of this awareness, more recently human and animal replacement valves are transplanted as a complete root, i.e. the whole aortic or pulmonary root of the patient is resected and replaced with a cylinder of the donor's homologous or heterologous tissue carrying the sigmoid valve. Both extremities of this natural replacement valved conduit are sutured to the remnant of the patient's tissues (see Kouchoukos N. T. New Eng J. Med. 1994;330:1). However, in this operation the correct sizing of the conduit in terms of diameter, length and orientation when suturing it, are essential if valve insufficiency is to be avoided. Distortions from the required dimensions and configuration not only can result in an immediate valve regurgitation but also in a progressive and slow deterioration of the transplanted valve which is subjected to deformation stresses. Because of the foregoing technical difficulties the use of these unsupported valves has not been universally accepted, and therefore many cardiac surgeons still primarily use mechanical or bioprosthetic devices for aortic valve replacement. The present design of an external support for a constructed or manufactured valved conduit, or for homologous or heterologous aortic or pulmonary tissue roots simplifies the surgical technique of their implantation and avoids distortions both at the time of its surgical implantation and during and after chest closure in the very precise geometry of the sigmoid valve complex.